The invention relates generally to wireless communication systems, and, more particularly, to a system and method for preassigning and reassigning a transmission channel based on channel characteristics, such as the interference and quality of a transmission channel using variable reassignment.
The increased demand for wireless (or cellular) communications has resulted in advanced communications systems capable of providing a high quality of service to more users. The systems that have emerged include analog systems such as Advanced Mobile Phone System (AMPS), and digital systems such as the Global System for Mobile Communications (GSM) and Digital AMPS (D-AMPS). To implement these systems, cellular based communication devices operate in interference-limited environments that rely on frequency reuse plans to maximize capacity and quality. Protocols such as the Frequency Division Multiple Access (FDMA) in analog systems, and the Time Division Multiple Access (TDMA) in digital systems are used for this purpose.
Because bandwidth is a limited resource (both in a physical and regulatory sense), schemes to divide the available network resources among as many subscribers as possible are often employed. One such scheme which seeks to maximize bandwidth allocation efficiency involves a combination TDMA and FDMA (TDMA/FDMA). For example, a 25 MHZ bandwidth can be divided into 124 carrier frequencies spaced 200 kHz apart, with one or more carrier frequencies assigned to each Base Station in the network. Each of the carrier frequencies, in turn, can be divided in time using a TDMA scheme to define logical channels. Channels, in turn, may be divided into dedicated subchannels which are allocated to a mobile station.
A traffic channel, also commonly called a communication or transmission channel, is used to carry speech and data (typically non-command) signals over the network. Traffic channels are defined in GSM using a multi-frame, or a group of 26 separate TDMA frames. With 26 frames, typically 24 will be used for speech or data, one as a control channel, and one will be unused. In other systems, such as Code Division Multiple Access (CDMA) or FDMA, other methods are used. In addition to dividing bandwidth among subscribers, traffic channels are assigned or allocated to incoming calls. Often, to increase channel efficiency, connection quality, and the number of potential channels available for subscribers within a base station operating area, an Idle Channel Measurement (ICM) algorithm is used to pre-select and assign channels for the transmission of speech and data signals.
Two methods of channel assignment are typically used in the industry: fixed channel assignment and dynamic channel assignment. Fixed channel assignment involves assigning a predetermined set of channels or frequencies to the base station or base station controller (BSC). If all the channels within the cell of the base station or BSC are occupied, and a call attempt is made, either the call is blocked, meaning the mobile handset does not receive service, or borrowing strategies utilize channels from neighboring cells to connect the call.
With dynamic channel assignment, channels are not allocated to a BSCs permanently. The Mobile Switching Center (MSC) distributes and redistributes frequencies among BSCs and base stations. The assignments remain fixed only for a period of time. Then, periodically, some measurements are made regarding channel utilization, and adjustments are made to the channel assignments as needed. The channels may be reallocated by a BSC each time a call request is made to more efficiently allocate channels among base stations.
A problem with prior art channel assignment algorithms and devices is that channel assignments are typically made based on the lowest interference level available even though medium interference channels would suffice given the channel quality between the mobile and the switching device. This results in an unbalanced use of traffic channels, and an increase in interference between channels. In addition, methods that assign channels solely on the basis of interference measurements may refuse connections for calls even when a lower quality channel is available to service a call, and could be implemented with no noticeable quality difference by the mobile station subscriber.
Furthermore, present systems of channel reallocation typically iterate to a best channel by searching adjacent channels one at a time. Iterating along adjacent channels to a channel with less interference requires constant processing. An assignment method that reduces overall system processing requirements, and more evenly allocates channels across the available transmission channels would be advantageous. The present invention provides such a system and method.
The present invention provides a method and system for decreasing the high processing requirements that result from prior art transmission channel allocation algorithms. Channels are preassigned and then reassigned using an algorithm that assigns service variably to a channel based on uplink interference and channel quality measurements. Reassignment may be achieved in one processing pass, rather than requiring multiple iterations. The present invention also solves the problem of channel crowding by initially preassigning service to a channel having a relatively high, but acceptable interference level, and then reassigning the service to a second channel only when necessary to improve channel quality.
Disclosed in one embodiment, is a method of assigning transmission channels to a service in a wireless communications network. The method comprises the step of preassigning a first transmission channel to the service, the transmission channel having an interference level less than a predetermined blocking threshold. Next, the service is reassigned to a second channel based on the channel quality and interference level. If reassignment is necessary, a second channel is selected so that the lower the channel quality of the first channel the lower the interference level of the second channel. A rule table defining bands of interference and categories of channel quality may be used to select the second channel. With variable reassignment the location of the interference band for the second channel may be a variable distance away from the interference band of the preassigned first channel.
The method may also be performed by assigning transmission channels according to a set of predefined interference bands and categories of channel quality. Variable reassignment segregates transmission channels in the network into a number of interference bands, ranging from a best interference band to a worst interference band as well as into categories of channel quality ranging from the lowest channel quality to highest channel quality. Channel quality can be measured as a Bit Error Rate (BER) or as a carrier to interference ratio (C/I) of the preassigned transmission channel. To maximize the number of channels utilized in the network, the step of preassigning could select the first channel as an available channel with the highest interference level less than a predetermined interference blocking threshold.
Also disclosed is a method of assigning a channel in a wireless communications network having a plurality of transmission channels. The method comprises the steps of measuring an interference level for each of the plurality of channels, assigning each of the plurality of channels to an interference band, preassigning a first channel to a service, measuring the channel quality of the first channel and reassigning the service from the first channel to a second channel when the channel quality of the first channel becomes unsatisfactory, such that the second channel is selected based on the interference level and channel quality of the first channel using variable reassignment.
Also disclosed is a device for use in a wireless communication network have a plurality of transmission channels, the device containing an algorithm for assigning transmission channels to a request for service. The algorithm comprises a discrimination function adapted to the determine the interference and channel quality of transmission channels in the network. A preassignment function is adapted to preassign a first transmission channel with an interference level less than a predetermined block and threshold to the service. The algorithm also includes a monitoring function adapted to continually monitor the channel quality of the preassigned transmission channel and a variable reassignment function that reassigns the service to a second transmission channel when the channel quality of the preassigned channel becomes unsatisfactory. The algorithm can include a segregation function configured to classify transmission channels according to bands of interference and categories of channel quality. The segregation function can be further adapted to create a rule table containing a configuration of the categories of channel quality and bands and interference, the rule table used by the variable reassignment function when reassigning a service to a second transmission channel.
The technical advantages of the present invention are numerous and include making the channel quality distribution in the system more uniform, reducing co-channel distance with increased transmission quality and capacity, and reducing system processing requirements of the communications network.